Black infants face the highest rates of prematurity in the U.S. (18%), have high rates of prematurity-associated wheezing illnesses, and tend to have lower vitamin D levels. The goal of this [comparative effectiveness] study is to identify a vit. D supplementation strategy that minimizes recurrent wheezing in infancy. Long recognized as important for bone health, a growing body of evidence suggests that vit. D may play a role in the regulation and development of many organ systems. The D pathway regulates lung inflammation and impacts morphogenesis, structure, and cell growth and survival in bronchial smooth muscle. Vit. D exposure has the potential to skew cytokine expression from a Th1 (less allergic) to a Th2 (more allergic) phenotype. Due to their developmental immaturity, preterm infants may be particularly vulnerable to any positive or negative effects of vit. D supplementation on the lung, airway, and immune system. Our preliminary data, supported by the literature, suggests that overly aggressive vit. D supplementation may inadvertently increase wheezing in infancy in black, but not white, preterm infants; however, vit. D deficiency could theoretically also increase wheezing via vulnerability to respiratory pathogens. The proposed study is a randomized clinical trial comparing the effect of two different enteral vitamin D supplementation strategies on recurrent wheezing in infancy in 300 black infants born preterm at 300/7-366/7 wks gestational age, a population for whom neither vit. D requirements nor optimal vit. D serum levels have been established. We will test two strategies: (I) sustained supplementation until 6 mo. of age adjusted for prematurity, and (II) cessation of supplementation when a minimum dietary intake of 200 IU/day is reached. The specific aims are to characterize the effect of each strategy on (aim 1) recurrent wheezing and (aim 2) [allergic sensitization] and atopy. We will (aim 3) explore the relationship between vit. D serum levels and recurrent wheezing. We hypothesize that strategy II will be more effective in promoting pulmonary health by minimizing recurrent wheezing, [allergic sensitization], and overall healthcare utilization, and will be sufficient to prevent clinical vit. D deficiency. We also hypothesize that optimal vit. D serum levels will be lower than the norms for other populations. Theoretical Association: Vit. D Status and Health. PUBLIC HEALTH RELEVANCE: The goal of this study is to identify a vitamin D supplementation strategy that best promotes the lung, immune, and overall health of black infants born preterm (30-36 weeks gestational age). This is a high risk population that seems to have unique vitamin D needs, and inappropriate supplementation may promote wheezing or allergy. The results of this study will help form nutritional recommendations for the approximately 100,000 black infants born at 30-36 weeks gestational age in the U.S. every year.